The epsilon4 allele of apolipoprotein E (APOE) is currently the major genetic risk factor for Alzheimer's disease and is associated with 25- 50% of late-onset AD. However, while the epsilon4 allele is generally recognized to play an important role in the pathogenesis of AD its mechanism of action remains controversial and is poorly understood. We and others have demonstrated in vitro that one potentially important mechanism of action involves its role in Abeta clearance. It is therefore important to determine whether it regulates Abeta clearance in an isoform specific manner in vivo. In addition to the epsilon 4 allele effect, we discovered in 1997 that increased plasma apoE levels are also associated with an increased risk of AD which is independent of APOE genotype and this finding dovetails with several reports that the -491 AA APOE promoter polymorphism exhibits increased APOE gene transcriptional activity and is associated with increased risk of AD. We have subsequently shown that the A-491 AA promoter polymorphism is associated with increased risk of AD in the Australian Caucasian population, and in turn, this polymorphism is associated with increased plasma and brain apoE levels. A major focus will involve the determination of whether the apoE isoform effects on Abeta clearance are modified by apoE concentration. The experiments in Specific Ami 1 will determine the rate of Abeta clearance from brain and peripheral tissues in the presence of various levels of epsilon2, epsilon3 or epsilon4 apoE isoform, whereas those in Specific Aim 2 will assess the effect of apoE isoform and levels on APP metabolism and Abeta function. Proposed methods will utilize tissue- and apoE isoform-specific transgenic mice, primary neuronal, glial and hepatocyte cultures, and sensitive apoE and Abeta assays.